Oxidation protection with improved water resistance for composites

ABSTRACT

Systems and methods for forming an oxidation protection system on a composite structure are provided. In various embodiments, the oxidation protection system comprises a boron-glass layer formed on the composite substrate and a silicon-glass layer formed over the boron-glass layer. Each of the boron-glass layer and the silicon-glass layer includes a glass former.

FIELD

The present disclosure relates generally to composites and, more specifically, to oxidation protection systems for carbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typically designed to minimize loss of carbon material due to oxidation at high temperature operating conditions, which include temperatures of 900° C. (1652° F.) or greater. Oxidation protection systems including layers of boron carbide and silicon carbide may reduce infiltration of oxygen and oxidation catalysts into the composite structure. However, such oxidation protection systems may exhibit hydrolytic instability, as diboron trioxide (B₂O₃), which may be formed during operation of the component at increased temperatures, is water soluble.

SUMMARY

A method for forming an oxidation protection system on a carbon-carbon composite structure is disclosed herein. In accordance with various embodiments, the method may comprise applying a boron slurry to the carbon-carbon composite structure, applying a silicon slurry to the carbon-carbon composite structure, and heating the carbon-carbon composite structure. The boron slurry may comprise a boron compound, a first glass compound, a first glass former, and a first carrier fluid. The silicon slurry may comprise a silicon compound, a second glass compound, a second glass former, and a second carrier fluid.

In various embodiments, the boron compound may comprise a mixture of boron carbide powder and boron nitride powder. In various embodiments, each of the first glass former and the second glass former may comprise colloidal silica.

In various embodiments, the method may further comprise applying a pretreatment composition to the carbon-carbon composite structure. The pretreatment composition may comprise at least one of aluminum oxide, silicon dioxide, or monoaluminium phosphate. In various embodiments, the boron slurry may further comprise aluminum oxide, and the silicon slurry may further comprise zirconium boride

In various embodiments, the boron nitride powder may form a greater weight percentage of the boron slurry than the boron carbide powder. In various embodiments, each of the boron slurry and the silicon slurry may further comprise a monoaluminium phosphate solution. In various embodiments, each of the first glass and the second glass may comprise borosilicate glass.

An oxidation protection system disposed on an outer surface of a substrate is also disclosed herein. In accordance with various embodiments, the oxidation protection system may comprise a boron-glass layer disposed over the outer surface and a silicon-glass layer disposed on the boron-glass layer. The boron-glass layer may comprise a boron compound, a first glass compound, and a first glass former. The silicon-glass layer may comprise a silicon compound, a second glass compound, and a second glass former.

In various embodiments, at least one of the first glass former or the second glass former may comprise silica, and at least one of the first glass compound or the second glass compound may comprise borosilicate glass.

In various embodiments, the boron compound may comprise a mixture of boron carbide powder and boron nitride powder. In various embodiments, a pretreatment layer may be formed between the boron-glass layer and the outer surface of the substrate. The pretreatment layer may include at least one of aluminum oxide, silicon dioxide, or monoaluminium phosphate.

In various embodiments, at least one of the boron-glass layer or the silicon-glass layer may include monoaluminium phosphate. In various embodiments, the boron-glass layer may further comprise aluminum oxide, and the silicon-glass layer may further comprise zirconium boride.

A method for forming an oxidation protection system on a brake disk is also disclosed herein. In accordance with various embodiments, the method may comprise forming a boron slurry by mixing a boron compound, a first glass compound, a first glass former, and a first carrier fluid; applying the boron slurry over a non-wear surface of the brake disk; forming a silicon slurry by mixing a silicon compound, second glass compound, a second glass forming, a second carrier fluid; applying the silicon slurry over the non-wear surface of the brake disk; and heating the brake disk at a first temperature.

In various embodiments, the method may further comprise drying the brake disk after applying the boron slurry to remove the first carrier fluid. In various embodiments, the method may further comprise drying the brake disk after applying the silicon slurry to remove the second carrier fluid.

In various embodiments, drying the brake disk after applying the silicon slurry may comprises heating the brake disk at a second temperature less than the first temperature. In various embodiments, the first glass former and the second glass former may each comprise colloidal silica. In various embodiments, the boron compound may comprise a mixture of boron carbide powder and boron nitride powder, and the boron nitride powder may form a greater weight percentage of the boron slurry than the boron carbide powder.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the detailed description and claims when considered in connection with the drawing figures, wherein like numerals denote like elements.

FIG. 1 illustrates a cross sectional view of an aircraft wheel braking assembly, in accordance with various embodiments;

FIG. 2 illustrates a method for forming an oxidation protection system on a composite structure, in accordance with various embodiments;

FIG. 3 illustrates a method for forming an oxidation protection system on a composite structure, in accordance with various embodiments; and

FIG. 4 illustrates experimental data obtained from testing oxidation protection systems, in accordance with various embodiments.

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to the accompanying drawings, which show embodiments by way of illustration. While these embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that logical and mechanical changes may be made without departing from the spirit and scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not for limitation. For example, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option.

With initial reference to FIG. 1 , aircraft wheel braking assembly 10 such as may be found on an aircraft, in accordance with various embodiments is illustrated. Aircraft wheel braking assembly may, for example, comprise a bogie axle 12, a wheel including a hub 16 and a wheel well 18, a web 20, a torque take-out assembly 22, one or more torque bars 24, a wheel rotational axis 26, a wheel well recess 28, an actuator 30, multiple brake rotors 32, multiple brake stators 34, a pressure plate 36, an end plate 38, a heat shield 40, multiple heat shield sections 42, multiple heat shield carriers 44, an air gap 46, multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52, multiple heat shield fasteners 53, multiple rotor lugs 54, and multiple stator slots 56.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed in wheel well recess 28 of wheel well 18. Rotors 32 are secured to torque bars 24 for rotation with wheel 14, while stators 34 are engaged with torque take-out assembly 22. At least one actuator 30 is operable to compress interleaved rotors 32 and stators 34 for stopping the aircraft. In this example, actuator 30 is shown as a hydraulically actuated piston, but many types of actuators are suitable, such as an electromechanical actuator. Pressure plate 36 and end plate 38 are disposed at opposite axial ends of the interleaved rotors 32 and stators 34. Rotors 32 and stators 34 can comprise any material suitable for friction disks, including ceramics or carbon materials, such as a carbon/carbon composite.

Through compression of interleaved rotors 32 and stators 34 between pressure plates 36 and end plate 38, the resulting frictional contact slows rotation of wheel 14. Torque take-out assembly 22 is secured to a stationary portion of the landing gear truck such as a bogie beam or other landing gear strut, such that torque take-out assembly 22 and stators 34 are prevented from rotating during braking of the aircraft.

The friction disks (e.g., rotors 32, stators 34, pressure plate 36, end plate 38) may be formed of carbon-carbon (C/C) composites having carbon fibers disposed in a carbon matrix). The C/C composites may operate as heat sinks to absorb large amounts of kinetic energy converted to heat during slowing of the aircraft. Heat shield 40 may reflect thermal energy away from wheel well 18 and back toward rotors 32 and stators 34.

In various embodiments, brake disks of aircraft wheel braking assembly 10 may reach operating temperatures in the range from about 100° C. (212° F.) up to about 900° C. (1652° F.), or higher (e.g., 1093° C. (2000° F.)). The high temperatures experienced by aircraft wheel braking assembly 10 can lead to loss of C/C composite material due to oxidation of carbon. For example, various C/C composite components of aircraft wheel braking assembly 10 may experience both catalytic oxidation and inherent thermal oxidation caused by heating the composite during operation. In various embodiments, rotors 32 and stators 34 may be heated to sufficiently high temperatures that may oxidize the carbon surfaces exposed to air. At elevated temperatures, infiltration of air and contaminants may cause internal oxidation and weakening, especially in and around brake rotor lugs 54 or stator slots 56 securing the friction disks to the respective torque bar 24 and torque take-out assembly 22. Because C/C composite components of aircraft wheel braking assembly 10 may retain heat for a substantial time period after slowing the aircraft, oxygen from the ambient atmosphere may react with the carbon matrix and/or carbon fibers to accelerate material loss. Further, damage to brake components may be caused by the oxidation enlargement of cracks around fibers or enlargement of cracks in a reaction-formed porous barrier coating (e.g., a silicon-based barrier coating) applied to the C/C composite.

Elements identified in severely oxidized regions of C/C composite brake components include potassium (K) and sodium (Na). These alkali contaminants may come into contact with aircraft brakes as part of cleaning or de-icing materials. Other sources include salt deposits left from seawater or sea spray. These and other contaminants (e.g. Ca, Fe, etc.) can penetrate and leave deposits in pores of C/C composite aircraft brakes, including the substrate and any reaction-formed porous barrier coating. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude.

In various embodiments, an oxidation protection system, as disclosed herein, may be applied to the various components of aircraft wheel braking assembly 10 for protecting the components from oxidation. However, it will be recognized that the oxidation protection systems and methods of forming the same, as disclosed herein, may be readily adapted to many parts in this and other braking assemblies, as well as to other C/C composite structures susceptible to oxidation losses from infiltration of atmospheric oxygen and/or catalytic contaminants.

In various embodiments, a method for limiting an oxidation reaction in a substrate (e.g., a C/C composite structure) may comprise forming an oxidation protection system on the composite structure. With reference to FIG. 2A, a method 200 for forming an oxidation protection system on a composite structure is illustrated. In accordance with various embodiments, method 200 may, for example, comprise applying an oxidation inhibiting composition to non-wearing surfaces of C/C composite brake components, such as non-wear surfaces 45 and/or lugs 54. Non-wear surface 45, as labeled in FIG. 1 , simply references an exemplary non-wear surface on a brake disk, but non-wear surfaces similar to non-wear surface 45 may be present on any brake disks (e.g., rotors 32, stators 34, pressure plate 36, end plate 38, or the like). In various embodiments, method 200 may be used on the back face of pressure plate 36 and/or end plate 38, an inner diameter (ID) surface of stators 34 including slots 56, as well as an outer diameter (OD) surface of rotors 32 including lugs 54. Method 200 may be performed on densified C/C composites. In this regard, method 200 may be performed after carbonization and densification of the C/C composite.

In various embodiments, method 200 may comprise forming a boron slurry (step 210). The boron slurry may be formed by combining a boron compound, a glass compound, and a silica (SiO₂) glass former with a carrier fluid (such as, for example, water). In various embodiments, the boron compound may comprise at least one boron-comprising refractory material (e.g., ceramic material). In various embodiments, the boron compound may comprise boron carbide, titanium diboride, boron nitride, zirconium boride, silicon hexaboride, elemental boron, and/or mixtures thereof.

In various embodiments, the boron slurry may comprise from about 3% to about 30% by weight boron compound, from about 5% to about 20% by weight boron compound, or from about 7% to about 18% by weight boron compound. As used in previous context only, the term “about” means±1.0 weight percent. In various embodiments, the boron slurry may comprise approximately 7.41% by weight boron compound. As used in this context only, the term “approximately” means±0.50 weight percent. In various embodiments, the boron slurry may comprise approximately 16.67% by weight boron compound. As used in this context only, the term “approximately” means±0.50 weight percent.

In various embodiments, the boron compound comprises a mixture of boron carbide powder and boron nitride powder. In various embodiments, boron nitride powder forms a greater weight percentage of the boron slurry as compared to the weight percentage formed by the boron carbide powder. In various embodiments, the boron carbide powder may form between about 4% and about 6% of the weight percentage of the boron slurry, and the boron nitride powder may form between about 10% and about 13% of the weight percentage of the boron slurry. As used in previous context only, the term “about” means±1.0 weight percent. In various embodiments, the boron carbide powder may form approximately 5% of the weight percentage of the boron slurry, and the boron nitride powder may form approximately 11.6% of the weight percentage of the boron slurry. As used in previous context only, the term “approximately” means±0.50 weight percent. In various embodiments, the boron carbide powder may form between about 1% and about 2.5% of the weight percentage of the boron slurry, and the boron nitride powder may form between about 4.5% and about 6.5% of the weight percentage of the boron slurry. As used in this context only, the term “about” means±1.0 weight percent. In various embodiments, the boron carbide powder may form approximately 1.85% of the weight percentage of the boron slurry, and the boron nitride powder may form approximately 5.56% of the weight percentage of the boron slurry. As used in previous context only, the term “approximately” means±0.50 weight percent.

In various embodiments, the boron compound is a powder (e.g., boron carbide powder, boron nitride powder, etc.). The boron compound powder may comprise particles having an average particle size of between about 100 nm and about 100 μm, between about 500 nm and about 50 μm, between about 1 μm and about 20 μm, between about 5 μm and about 10 μm, and/or between about 500 nm and about 1 μm. As used in the previous context only, the term “about” means±10% of the associated value. In various embodiments, the boron compound powder may comprise particles having an average particle size of approximately 0.7 μm. As used in the previous context only, the term “approximately” means±0.25 μm. In various embodiments, the boron compound powder may comprise particles having an average particle size of approximately 9.3 μm. As used in the previous context only, the term “approximately” means±0.25

The glass compound of the boron slurry may comprise borosilicate glass, borophosphate, quartz, aluminosilicate, boroaluminosilicate, and/or any other suitable glass compound. The glass compound may be in the form of a glass frit or other pulverized form. In various embodiments, the glass compound is borosilicate glass. In various embodiments, the borosilicate glass may comprise in weight percentage 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% sodium oxide (Na₂O), and may have a CTE of 3.3×10⁻⁶ cm/C, a working point of 2286° F. (1252° C.), and an annealing point of 1040° F. (560° C.). In various embodiments, the boron slurry may comprise between about 5% and about 50% by weight glass compound, between about 10% and about 35% by weight glass compound, between about 24% and about 26% by weight glass compound, or between about 27% and about 29% by weight glass compound. As used in previous context only, the term “about” means±1.0 weight percent. In various embodiments, the glass compound may form approximately 25% of the weight percentage of the boron slurry. As used in the previous context only, the term “approximately” means±0.50 weight percent. In various embodiments, the glass compound may form approximately 27.78% of the weight percentage of the boron slurry. As used in the previous context only, the term “approximately” means±0.50 weight percent.

In various embodiments, the silica glass former may include colloidal silica, metal silicates, alkyl silicates, and/or amorphous or crystalline silica. In various embodiments, the silica glass former is a colloidal silica suspension having 40% by weight silica. The silica glass former may form between about 10% and about 40%, between about 20% and about 30%, or between about 24% and about 28% of the weight percentage of the boron slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the silica glass former may form approximately 25.0% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the silica glass former may form approximately 27.78% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the boron slurry may also comprise monoaluminium phosphate. The monoaluminium phosphate may be in the form of a solution (e.g., monoaluminium phosphate and a carrier fluid) of any suitable make-up. In various embodiments, the monoaluminium phosphate solution may comprise about 50% by weight monoaluminium phosphate and about 50% by weight carrier fluid (e.g., water). In the previous context only, the term “about” means±10.0 weight percent. In various embodiments, the monoaluminium phosphate solution may form between about 1% and about 10%, between about 2% and about 5%, or between about 3% and about 4% of the weight percentage of the boron slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the monoaluminium phosphate solution may form approximately 3.33% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the monoaluminium phosphate solution may form approximately 3.7% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the boron slurry may also comprise a glass modifier such as, for example, aluminum oxide. In various embodiments, the glass modifier may form between about 0.5% and about 10%, between about 1% and about 5%, or between about 1% and about 2% of the weight percentage of the boron slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the glass modifier may form approximately 1.67% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the glass modifier may form approximately 1.85% of the weight percentage of the boron slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the boron slurry may comprise, in weight percentage, 5.0% boron carbide powder, 11.67% boron nitride powder, 25% borosilicate glass, 3.33% monoaluminium phosphate solution, 25% colloidal silica suspension, 1.67% aluminum oxide, and 28.33% water. The borosilicate glass may comprise in weight percentage 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% Na₂O, and may have a CTE of 3.3×10⁻⁶ cm/C, a working point of 2286° F. (1252° C.), and an annealing point of 1040° F. (560° C.). The colloidal silica suspension may be 40% by weight silica. The monoaluminium phosphate solution may be 50% by weight monoaluminium phosphate and 50% by weight carrier fluid. The colloidal silica suspension may be 40% by weight silica.

In accordance with various embodiments, method 200 further comprises applying the boron slurry to a composite structure (step 220). Applying the boron slurry may comprise, for example, spraying or brushing the boron slurry to an outer surface of the composite structure. Any suitable manner of applying the boron slurry to the composite structure is within the scope of the present disclosure. As referenced herein, the composite structure may refer to a C/C composite structure. In accordance with various embodiments, the boron slurry may be applied directly on (i.e., in physical contact with) the surface of the composite structure.

In various embodiments, method 200 may comprise forming a silicon slurry (step 230) by combining a silicon compound, a glass compound, and a silica glass former compound with a carrier fluid (such as, for example, water). In various embodiments, the silicon compound may comprise at least one silicon-comprising refractory material (e.g., ceramic material). In various embodiments, the silicon compound may comprise silicon carbide, a silicide compound, silicon, silicon dioxide, silicon carbonitride, or combinations thereof.

In various embodiments, the silicon slurry may comprise from about 5% to about 30% by weight silicon compound, from about 10% to about 20% by weight silicon compound, from about 9% to about 10% by weight silicon compound, or from about 18% to about 20% by weight silicon compound. As used in previous context only, the term “about” means±1.0 weight percent. In various embodiments, the silicon slurry may comprise approximately 18.94% by weight silicon compound. As used in previous context only, the term “approximately” means±0.50 weight percent. In various embodiments, the silicon slurry may comprise approximately 10.08% by weight silicon compound. As used in the previous context only, the term “approximately” means±0.50 weight percent.

In various embodiments, the silicon compound is a powder (e.g., silicon powder or silicon carbide powder). The silicon compound powder may comprise particles having an average particle size of between about 100 nm and 50 μm, between 500 nm and 20 μm, between 500 nm and 1.5 μm, or between 16 μm and 18 μm. As used in previous context only, the term “about” means±1.0 μm. In various embodiments, the silicon compound may comprise particles having an average particle size of approximately 17 μm. As used in the previous context only, the term “about” means±0.25 μm. In various embodiments, the silicon compound may comprise particles having an average particle size of approximately 1.0 μm. As used in the previous context only, the term “about” means±0.25 μm.

The glass compound of the silicon slurry may comprise borosilicate glass, borophosphate, quartz, aluminosilicate, boroaluminosilicate, and/or any other suitable glass compound. The glass compound may be in the form of a glass frit or other pulverized form. In various embodiments, the glass compound is borosilicate glass. In various embodiments, the borosilicate glass may comprise in weight percentage 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% Na₂O, and may have a CTE of 3.3×10⁻⁶ cm/C, a working point of 2286° F. (1252° C.), and an annealing point of 1040° F. (560° C.). In various embodiments, the silicon slurry may comprise between about 5% and about 50% by weight glass compound, between about 10% and about 30% by weight glass compound, between about 22% and about 24% by weight glass compound, or between about 24% and about 26% by weight glass compound. As used in previous context only, the term “about” means±1.0 weight percent. In various embodiments, the glass compound may form approximately 22.73% of the weight percentage of the silicon slurry. As used in the previous context only, the term “approximately” means±0.50 weight percent. In various embodiments, the glass compound may form approximately 25.21% of the weight percentage of the silicon slurry. As used in the previous context only, the term “approximately” means±0.50 weight percent.

In various embodiments, the silica glass former may include colloidal silica, metal silicates, alkyl silicates, and/or elemental silica. In various embodiments, the silica glass former is a colloidal silica suspension having 40% by weight silica. The silica glass former may form between about 10% and about 40%, between about 20% and about 30%, between about 22% and about 24%, or between about 24% and 26% of the weight percentage of the silicon slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the silica glass former may form approximately 22.73% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the silica glass former may form approximately 25.21% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the silicon slurry may also comprise monoaluminium phosphate. The monoaluminium phosphate may be in the form of a solution (e.g., monoaluminium phosphate and a carrier fluid) of any suitable make-up. In various embodiments, the monoaluminium phosphate solution may comprise about 50% by weight monoaluminium phosphate and about 50% by weight carrier fluid (e.g., water). In the previous context only, the term “about” means±10 weight percent. In various embodiments, the monoaluminium phosphate solution may form between about 1% and about 10%, between about 2% and about 5%, or between about 3% and about 4% of the weight percentage of the silicon slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the monoaluminium phosphate solution may form approximately 3.03% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the monoaluminium phosphate solution may form approximately 3.36% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the silicon slurry may also comprise a glass modifier such as, for example, zirconium boride. In various embodiments, the glass modifier may form between about 1.0% and about 15%, between about 3% and about 10%, or between about 5% and about 7% of the weight percentage of the silicon slurry. As used the previous context only, the term “about” means±1.0 weight percent. In various embodiments, the glass modifier may form approximately 6.06% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent. In various embodiments, the glass modifier may form approximately 6.72% of the weight percentage of the silicon slurry. As used the previous context only, the term “approximately” means±0.5 weight percent.

In various embodiments, the silicon slurry may comprise, in weight percentage, 8.94% silicon carbide powder, 22.73% borosilicate glass, 3.03% monoaluminium phosphate solution, 22.73% colloidal silica suspension, 6.06% zirconium boride, and 26.52% water. The borosilicate glass may comprise in weight percentage 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% Na₂O, and may have a CTE of 3.3×10⁻⁶ cm/C, a working point of 2286° F. (1252° C.), and an annealing point of 1040° F. (560° C.). The colloidal silica suspension may be 40% by weight silica. The monoaluminium phosphate solution may be 50% by weight monoaluminium phosphate and 50% by weight carrier fluid.

In various embodiments, method 200 further comprises applying the silicon slurry to the composite structure (step 240). Applying the silicon slurry may comprise, for example, spraying or brushing the boron slurry to an outer surface of the composite structure. Any suitable manner of applying the silicon slurry to the composite structure is within the scope of the present disclosure. As referenced herein, the composite structure may refer to a C/C composite structure.

In various embodiments, method 200 may further comprise performing a high temperature cure (step 250) to form a boron-glass layer on the composite structure and a silicon-glass layer on the boron-glass layer. Step 250 may include heating the composite structure at a relatively high temperature, for example, a temperature of about 1500° F. (816° C.) to about 1700° F. (927° C.), or about 1650° F. (899° C.), wherein the term “about” in previous context only means±25° F. (±4° C.)). Step 250 may include heating the composite structure for about 5 minutes to about 8 hours, about 30 minutes to about 5 hours, or about 2 hours, wherein the term “about” in this context only means±10% of the associated value.

Not to be bound by theory, it is believed that boron components may become oxidized during service at high temperatures (e.g., temperatures greater than 1300° F. (704° C.)), thereby forming boron trioxide. The boron trioxide may come into contact with the silica glass former or oxidized silicon components to form a borosilicate in situ, providing a method of self-healing. For a boron-silicon oxidation protection system, the probability of boron trioxide reacting with oxidized silicon compounds is kinetically controlled and influenced by the amount of each component, surface area, aspect ratio, etc. Boron trioxide is also volatile, especially when hydrated to form boric acid, and may be lost during extended service time. Method 200 tends to increase the probability of self-healing borosilicate formation by creating a layer of silicon, glass, and glass former over the boron-glass layer that can reduce boron trioxide transportation in water prior to volatilization. The silica glass former in the boron-glass layer and in the silicon-glass layer may also react with the boron trioxide forming, for example, borosilicate glass, laminboard glass, borophosphate glass, etc. in the boron-glass and/or silicon-glass layers. The silica reacting with the boron trioxide, tends to decrease or eliminate unreacted boron trioxide, thereby increasing the water stability of the oxidation protection system.

In various embodiments and with reference to FIG. 3 , a method 300 of forming an oxidation protection system on a composite structure is illustrated. In addition to steps 210, 220, 230, 240, and 250 from method 200 in FIG. 2 , method 300 may comprise applying a pretreatment composition to the composite structure (step 205) prior to applying the boron slurry (step 220). Step 205 may, for example, comprise applying a pretreatment composition to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly 10 (FIG. 1 ). In various embodiments, the pretreatment composition comprises an aluminum oxide, a silicon dioxide, or combinations thereof in water. For example, the aluminum oxide may comprise a nanoparticle dispersion of aluminum oxide and/or a nanoparticle dispersion of silicon dioxide. The pretreatment composition may further comprise a surfactant or a wetting agent. In various embodiments, after applying the pretreatment composition, the composite structure is heated to remove water and fix the aluminum oxide and/or silicon dioxide in place. For example, the composite structure may be heated between about 100° C. (212° F.) and 200° C. (392° F.) or between 100° C. (212° F.) and 150° C. (302° F.).

In various embodiments, the pretreatment composition may comprise monoaluminium phosphate. The monoaluminium phosphate may be in the form of a solution (e.g., monoaluminium phosphate and a carrier fluid) of any suitable make-up. In various embodiments, the monoaluminium phosphate solution may comprise about 50% by weight monoaluminium phosphate and about 50% by weight carrier fluid (e.g., water). In the previous context only, the term “about” means±10 weight percent. In various embodiments, after applying the pretreatment composition, the composite structure is heated to remove carrier fluid and fix the monoaluminium phosphate over the composite structure. In accordance with various embodiments, method 300 may include applying the boron slurry (step 220) after heating the composite structure to remove the carrier fluid (e.g., water) of the pretreatment composition.

Not to be bound by theory, it is believed that the aluminum oxide and/or silicon dioxide and/or monoaluminium phosphate from the pretreatment composition may react with the boron trioxide, which may form in the boron-glass layer at elevated temperatures. The aluminum oxide and/or silicon dioxide and/or monoaluminium phosphate reacting with the boron trioxide, tends to decrease or eliminate unreacted boron trioxide, thereby increasing the water stability of the oxidation protection system. Additionally, the aluminum oxide and/or silicon dioxide and/or monoaluminium phosphate tends to increase the temperature range of the oxidation protection system. For example, a monoaluminium phosphate lay tends to provide oxidation protection at low temperatures (e.g., temperature of up to 482° C. (900° F.)), while boron carbide, boron nitride, and silicon-carbide may provide oxidation protection at high temperatures (e.g., temperature of greater than 482° C. (900° F.)).

With continued reference to FIG. 3 , in various embodiments, method 300 may further include drying the composite structure after applying the boron slurry (step 225). Step 225 may be performed prior to applying the silicon slurry (i.e., prior to step 240). Step 225 may include heating the composite structure at a relatively low temperature (for example, a temperature between about 250° F. (121° C.) and about 350° F. (177° C.), or of about 300° F. (149° C.) to remove the carrier fluid of the boron slurry. In the previous context only, the term “about” means±25° F. (4° C.)). Step 225 may include heating the composite structure for about 5 minutes to about 8 hours, about 10 minutes to about 2 hours, or about 1 hour, wherein the term “about” in this context only means±10% of the associated value.

In various embodiments, method 300 may further comprise drying the composite structure after applying the silicon slurry (step 245). Step 245 may include heating the composite structure at a relatively low temperature (for example, a temperature between about 250° F. (121° C.) and about 350° F. (177° C.), or of about 300° F. (149° C.) to remove the carrier fluid of the silicon slurry. Step 245 may be followed step 250, wherein the composite structure is heated at a higher temperature, relative to the temperature in step 245. For example, in step 245, the composite structure may undergo a first heat treatment at a first temperature of about 250° F. (121° C.) and about 350° F. (177° C.). In step 250, the composite structure may undergo a second heat treatment at a second temperature of about 1600° F. (871° C.) to about 1700° F. (927° C.). In various embodiments, the first temperature may be about 300° F. (149° C.) and the second temperature may about 1650° F. (899° C.). As used in the previous context only, the term “about” means±25° F. (4° C.). In various embodiments, the second temperature (i.e., the temperature of step 250) is selected to be below the working point of the glass compound in the boron and silicon slurries, for example, below the working point of the borosilicate glass.

In various embodiments, step 250 may be performed in an inert environment, such as under a blanket of inert or less reactive gas (e.g., nitrogen (N₂), argon, other noble gases, and the like). In various embodiments, the composite structure may be heated prior to application of the pretreatment composition and/or prior to application of the boron slurry and/or prior to application of the silicon slurry to aid in the penetration of the pretreatment composition and/or slurry. The temperature rise may be controlled at a rate that removes water without boiling and provides temperature uniformity throughout the composite structure.

TABLE 1 illustrates a first oxidation protection system (OPS) A and a second OPS B formed in accordance with the methods and compositions disclosed herein. Each of OPS A and OPS B is formed by applying a boron slurry including a glass former (e.g., colloidal silica) and a silicon slurry including a glass former (e.g., colloidal silica) to a C/C composite Each numerical value in TABLE 1 is the weight percentage of the component in the respective slurry. BSG 7740 is a borosilicate glass comprising 13% B₂O₃, 61% SiO₂, 2% Al₂O₃, and 4% Na₂O. Ludox AS-40 is a colloidal silica having 40% by weight silica. The monoaluminium phosphate is a solution of 50% by weight monoaluminium phosphate and 50% by weight water.

TABLE 1 Slurry Components OPS A OPS B Boron Slurry Wt % Wt % Boron Carbide (B₄C) 5.00 1.85 Boron Nitride (BN) 11.67 5.56 BSG 7740 25.00 27.78 Ludox AS-40 (40% colloidal silica) 25.00 27.78 Monoaluminium Phosphate (50% sol'n) 3.33 3.70 Aluminum Oxide (Al₂O₃) 1.67 1.85 Water 28.33 31.48 Silicon Slurry Wt % Wt % Silicon Carbide (SiC) 18.94 18.94 BSG 7740 22.73 22.73 Ludox AS-40 (40% colloidal silica) 22.73 22.73 Monoaluminium Phosphate (50% sol'n) 3.03 3.03 Zirconium Boride (ZrB₂) 6.06 6.06 Water 26.52 26.52

TABLE 2 illustrates an OPS C which was formed by applying a boron-glass slurry and a silicon-glass slurry one after the other to a C/C composite to form an oxidation protection system having a boron-glass layer and a silicon-glass layer. Each numerical value in TABLE 2 is the weight percentage of particular substance in the respective slurry.

TABLE 2 Slurry Component OPS C Boron Slurry Wt % Boron Carbide (B₄C) 45.00 BSG 7740 10.00 Water 55.00 Silicon Slurry Wt % Carbide (SiC) 45.00 BSG 7740 10.00 Water 55.00

TABLE 1, TABLE 2, and FIG. 4 may allow evaluation of an oxidation protection system comprising a boron-glass layer including a glass former, a silicon-glass layer including a glass forming, as described herein, versus an oxidation protection system including a boron-glass layer without a glass former and a silicon-glass layer without a glass former. In FIG. 4 , percent weight loss from the C/C composite is shown on the y-axis and exposure time is shown on the x-axis. A line 402 at hour five represents the point in time at which the C/C composites were exposed to water.

To form the OPS A, the performance of which is reflected by data set 404 in FIG. 4 , the boron slurry was applied to a first carbon-carbon composite coupon, the first carbon-carbon composite structure coupon was dried at 300° F. (149° C.). After drying for 1.0 hours the silicon slurry was applied. The first carbon-carbon composite coupon was then dried at 300° F. (149° C.). After drying for 1.0 hour, a high temperature cure was performed. The high temperature cure included heating the first carbon-carbon composite coupon at a temperature of 1650° F. (899° C.) for 2 hours.

To form the OPS B, the performance of which is reflected by data set 406 in FIG. 4 , the boron slurry was applied to a second carbon-carbon composite coupon, the second carbon-carbon composite coupon was dried at 300° F. (149° C.). After drying for 1.0 hours the silicon slurry was applied. The second carbon-carbon composite coupon was then dried at 300° F. (149° C.). After drying for 1.0 hours, a high temperature cure was performed. The high temperature cure included heating the second carbon-carbon composite coupon at a temperature of 1650° F. (899° C.) for 2 hours.

To form the OPS C, the performance of which is reflected by data set 408 in FIG. 4 , the boron slurry was applied to a third carbon-carbon composite coupon, the third carbon-carbon composite structure coupon was dried at 300° F. (149° C.). After drying for 1.0 hours the silicon slurry was applied. The third carbon-carbon composite coupon was then dried at 300° F. (149° C.). After drying for 1.0 hour, a high temperature cure was performed. The high temperature cure included heating the third carbon-carbon composite coupon at a temperature of 1650° F. (899° C.) for 2 hours.

After preparing each of OPS A, OPS B, and OPS C, the coupons were subjected to isothermal oxidation testing at 1200° F. (649° C.) over a period of hours while monitoring mass loss. At hour 5.0 the coupons were exposed to water.

As can be seen in graph 400 of FIG. 4 , the oxidation protection systems formed using a boron slurry and a silicon slurry that each include a glass former, reflected by data set 404 and data set 406, exhibited a significant decrease in weight loss after exposure to water at hour five as compared to the oxidation protection system where the boron and silicon slurries did not include a glass former, reflected by data set 408. In particular, the slope of data sets 404 and 406 between hour 5 and hour 8 is relatively flat (e.g., the weight loss increases by less than 1.0%), whereas the slope of data set 408 between hour 5 and hour 8 is relatively steep (e.g., the weight loss increases by more than 3.0%). Graph 400 indicates that the oxidation protection systems formed using a boron slurry and a silicon slurry that each include a glass former, such as colloidal silica, can increase the water stability of the oxidation protection system.

Benefits and other advantages have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments.

Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is intended to invoke 35 U.S.C. 112(f), unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 

What is claimed is:
 1. A method for forming an oxidation protection system on a carbon-carbon composite structure, comprising: applying a boron slurry to the carbon-carbon composite structure, wherein the boron slurry comprises a boron compound, a first glass compound, a first glass former, and a first carrier fluid; applying a silicon slurry to the carbon-carbon composite structure, wherein the silicon slurry comprises a silicon compound, a second glass compound, a second glass former, and a second carrier fluid; and heating the carbon-carbon composite structure.
 2. The method of claim 1, wherein the boron compound comprises a mixture of boron carbide powder and boron nitride powder.
 3. The method of claim 2, wherein each of the first glass former and the second glass former comprises colloidal silica.
 4. The method of claim 3, further comprising applying a pretreatment composition to the carbon-carbon composite structure, the pretreatment composition comprising at least one of aluminum oxide, silicon dioxide, or monoaluminium phosphate.
 5. The method of claim 4, wherein the boron slurry further comprises aluminum oxide, and wherein the silicon slurry further comprises zirconium boride.
 6. The method of claim 5, wherein the boron nitride powder forms a greater weight percentage of the boron slurry than the boron carbide powder.
 7. The method of claim 6, wherein each of the boron slurry and the silicon slurry further comprises a monoaluminium phosphate solution.
 8. The method of claim 7, wherein each of the first glass and the second glass comprises borosilicate glass.
 9. An oxidation protection system disposed on an outer surface of a substrate, the oxidation protection system, comprising: a boron-glass layer disposed over the outer surface, the boron-glass layer comprising a boron compound, a first glass compound, and a first glass former; and a silicon-glass layer disposed on the boron-glass layer, the silicon-glass layer comprising a silicon compound, a second glass compound, and a second glass former.
 10. The oxidation protection system of claim 9, wherein at least one of the first glass former or the second glass former comprises silica, and wherein at least one of the first glass compound or the second glass compound comprises borosilicate glass.
 11. The oxidation protection system of claim 10, wherein the boron compound comprises a mixture of boron carbide powder and boron nitride powder.
 12. The oxidation protection system of claim 10, further comprising a pretreatment layer formed between the boron-glass layer and the outer surface of the substrate, the pretreatment layer including at least one of aluminum oxide, silicon dioxide, or monoaluminium phosphate.
 13. The oxidation protection system of claim 10, wherein at least one of the boron-glass layer or the silicon-glass layer includes monoaluminium phosphate.
 14. The oxidation protection system of claim 10, wherein the boron-glass layer further comprises aluminum oxide, and wherein the silicon-glass layer further comprises zirconium boride.
 15. A method for forming an oxidation protection system on a brake disk, comprising: forming a boron slurry by mixing a boron compound, a first glass compound, a first glass former, and a first carrier fluid; applying the boron slurry over a non-wear surface of the brake disk; forming a silicon slurry by mixing a silicon compound, second glass compound, a second glass forming, a second carrier fluid; applying the silicon slurry over the non-wear surface of the brake disk; and heating the brake disk at a first temperature.
 16. The method of claim 15, further comprising drying the brake disk after applying the boron slurry to remove the first carrier fluid.
 17. The method of claim 16, further comprising drying the brake disk after applying the silicon slurry to remove the second carrier fluid.
 18. The method of claim 16, wherein drying the brake disk after applying the silicon slurry comprises heating the brake disk at a second temperature less than the first temperature.
 19. The method of claim 18, wherein the first glass former and the second glass former each comprise colloidal silica.
 20. The method of claim 19, wherein the boron compound comprise a mixture of boron carbide powder and boron nitride powder, and wherein the boron nitride powder forms a greater weight percentage of the boron slurry than the boron carbide powder. 